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Flow cytometry is a technology that measures simultaneously both optical and fluorescence properties of single cells or any other particle including cell organelles (i.e. nuclei, chromosome etc.), microorganisms, and latex beads. This technology also exploits bright beams of light (usually from laser) to directly disclose various aspects i.e. size, granularity (internal complexity of cell) including relative fluorescence intensity of cells by the way of light is scattered or indirectly by coupling wth fluorescent probes to cell components or suspended particle. Some components like dichroic mirror, optical filter, photomultiplier tube, detectors etc. that played a crucial role in flow cytometry. Therefore, light scattering properties and measurement of fluorescence through flow cytometry is widely used in life science laboratories including preclinical and clinical research for many applications such as immunological, entomological, pharmaceutical etc. This review provides information about the optical system that are used in flow cytometry and showed various biological applications.